Final Build critiques and how big of a PSU do i need?

[a67codkiller]

Ok. so this summer im going to build my first gaming PC and this is what i have picked out for a ~1300 budget:

Case: CM scout
mobo: Asus Maximus V LGA 1155 Intel Z77
Ram: 8 gigs
G. Card: Sapphire 7850 oc
Processor: i5-3570K
Asus DVD drive
1 HDD (7200)
Asus 23inch monitor
mouse and keyboard

How big of a PSU do i need? im thinking around 650 but im a noob at this stuff.

 

[Rugger]

A good quality 500W PSU is sufficient for your build, specifically an HD 7850 requires a 500W PSU with 28A on the 12V rails and one 75W 6-pin PCIe power connector.

 

[a67codkiller]

Thanks for the advice! but what if i want to Xfire? is there any harm in buying a bigger psu for down the road?